Gold plating electrolyte

ABSTRACT

A GOLD PLATING ELECTROLYTE FOR PROVIDING GOLD DEPOSITS WITH CONTROLLED HARDNESS BELOW 130 KNOOP, WHICH CONTAINS GOLD AS THE SULFITE COMPLEX, AND ALKALI METAL SULFITE, A BUFFERING AND CONDUCTING SALT, AND THE COMBINATION OF A POLYCARBOXYLIC ACID AND A SEMI-METAL BRIGHTENER.

States Patent Office 3,776,822 Patented Dec. 4, 1973 3,776,822 GOLD PLATING ELECTROLYTE Kenneth D. Baker, Somerville, N..l., assignor to Engelhard Minerals & Chemicals Corporation No Drawing. Filed Mar. 27, 1972, Ser. No. 238,615 Int. Cl. C23]: 5/28 US; Cl. 204-46 5 Claims ABSTRACT OF THE DISCLOSURE A gold plating electrolyte for providing gold deposits with controlled hardness below 130 Knoop, which contains gold as the sulfite complex, an alkali metal sulfite, a buffering and conducting salt, and the combination of a polycarboxylic acid and a semi-metal brightener.

BACKGROUND OF THE INVENTION Gold has been electrodeposited from sulfite gold complexes of alkali metals as early as 1842 (British Patent No. 9,431) and more recently United States Patents No. 3,057,789 and No. 3,475,292 have been granted for plating solutions containing sulfite gold complexes to give golddeposits which have a hardness of 130 to 200' Knoop. It has been discovered according to this invention that with the addition of a controlled quantity of a polycarboxylic acid together with a soluble semi-metal that bright, ductile pure deposits are obtained with a controlled hardness below 130 Knoop, and that by altering the quantities of the aforesaid acid and semi-metal deposits having a hardness below 90 Knoop are obtained.

SUMMARY OF THE INVENTION "In accordance with the invention there is provided a gold plating electrolyte for providing gold deposits with controlled hardness below 130 Knoop, which contains gold as the sulfite complex, an alkali metal sulfite, a buffering and conducting salt and the combination of a polycarboxylic acid together with a semi-metal brightener. The addition of a controlled amount of the polycarboxylic acid and the semi-metal brightener provide for bright, ductile pure gold deposits having a hardness below 130 Knoop, and by altering the quantities of the aforesaid acid and semi-metal, deposits having a hardness below 90 Knoop are obtained.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS The gold electrolyte according to the invention contains:

Grams/liter Gold as the sulfite complex 4-30 Alkali metal or ammonium buffering salt -200 Alkali metal sulfite 10-150 Polycarboxylic acid 0.025-1.5

Semi-metal brightener (as metal) 00025-015 Sufficient alkali metal salt to adjust the pH of the solution to 8-11 The gold sulfite complex is selected from the group consisting of potassium gold sulfite, sodium gold sulfite and ammonium gold sulfite.

Examples of the alkali metal or ammonium buffering salts are alkali metal or ammonium citrates, borates, phosphates, sulfamates and pyrophosphates.

The alkali metal sulfites employed are sodium or potassium sulfites.

Examples of the polycarboxylic acids employed are which are often used as bulfering or conducting media in gold electrolytes.

Examples of the soluble salts of the semi-metal employed are the soluble salts of antimony, arsenic, selenium and tellurium such as potassium antimony tartrate, arsenic trioxide, selenium as selenic acid neutralized with an alkali such as potassium or sodium hydroxide, and tellurium as a soluble salt such as tellurium dioxide dissolvedin potassium or sodium hydroxide.

The preferred combination of a polycarboxylic acid and semi-metal is oxalic acid and arsenic trioxide. The brightening hardness control solution is prepared by warming to dissolve the arsenic trioxide in an aqueous solution of oxalic acid. It has been determined that by increasing or decreasing the concentration of oxalic acid in the brightener or electrolyte, with, for example, the same concentration of arsenic, electrodeposits of varying hardness are obtained. It is preferred that the oxalic acid be added together with the brightening semi-metal.

The following examples further illustrate the invention:

EXAMPLE 1 Solution A.Into an amount of water suflicient to form one liter of solution is dissolved: 50 grams oxalic acid crystals and 10 grams arsenic as arsenic trioxide.

Electrolyte:

Gold as the sulfite complex-10 grams/liter Potassium citrate-70 grams/liter Sodium or potassium sulfite-25 grams/liter Solution A-3 mls./ liter pH adjusted to 9.0-9.5 with potassium hydroxide The hardness of the deposit obtained at 5 ASF (amperes per square foot) using a 25-gram load and a deposit thickness of over 2 mils was between 94 and 123 Knoop. The deposit was bright.

EXAMPLE 2 Solution B.Into an amount of water sufiicient to form one liter of solution is dissolved: grams oxalic acid crystals and 10 grams arsenic as arsenic trioxide.

Electrolyte Gold as the sulfite complex-10 grams/liter Potassium citrate-70 grams/liter Sodium or potassium sulfite-25 grams/liter Solution B-3 mls./liter pH adjusted to 9.0-9.5 with potassium hydroxide The hardness of the deposit obtained at 5 ASF using a 25-gram load and a deposit thickness of over 2 mils was between 76 and 85 Knoop. The deposit was bright.

EXAMPLE 3 Solution C.Into an amount of water sufficient to form one liter of solution is dissolved: 10 grams arsenic as arsenic trioxide. a V

Electrolyte:

Gold as the sulfite complex-10 grams/liter Potassium citrate-70 grams/ liter Sodium or potassium sulfite25 grams/liter Solution C3 mls./ liter pH adjusted to 9.0-9.5 with potassium hydroxide The hardness of the deposit obtained at 5 ASF using a 25-gram load and a deposit thickness of over 2 mils was between and Knoop. The deposit was bright.

EXAMPLE 4 Solution D.Into an amount of water sufficient to form one liter of solution is dissolved: 100 grams oxalic acid.

3 Electrolyte:

Gold as the sulfite complexl grams/liter Potassium citrate70 grams/liter Sodium or potassium sulfite-25 grams/liter Solution D3 mls./liter pH adjusted to 9.0-9.5 with potassium hydroxide The deposit obtained at ASP was brown and did not appear to be very compact. No hardness figure was obtained for the deposit.

The polycarboxylic acid and semi-metal brightener combination is added to the solution in the range of 4:1 to 20:1 ratio of polycarboxylic acid to semi-metal. For example the ratio may be 5:1 which gives the deposit a hardness of 90 to 130 Knoop and, preferably, the ratio may be :1 which gives the deposit a hardness of below 90 Knoop.

The quantity of such additive in the electrolyte is dependent upon the speed at which the electrolyte is operated. The following example illustrates the use of the additive in an electrolyte which is formulated to operate at substantially high current density where the current density is higher than given in Examples 1-4. The increase in the concentration of gold and potassium citrate is necessary to operate the electrolyte at such higher current density.

EXAMPLE 5 Gold as sulfite complex grams/liter Potassium citrate90 grams/liter Sodium or potassium sulfite grams/liter Solution A (form Example 1)6 mls./liter pH adjusted to 9.0-9.5 with potassium hydroxide The above electrolyte gives soft pure gold deposits with a hardness of 95 to 100 Knoop at 20 ASP.

It should be noted that the above-mentioned ratios of polycarboxylic acid to semi-metal salt does not exhibit the same reduction of hardness from an electrolyte using an alkali metal gold cyanide as the source of gold. This is illustrated in the following example.

EXAMPLE 6 Gold as potassium gold cyanide-10 grams/liter Potassium citrate-70 grams/liter Potassium dihydrogen phosphate grams/liter Solution A (from Example 1)3 mls./liter pH adjusted to 6.5 with H PO or KOH The hardness of the deposit plated at 5 ASP was 140 Knoop.

It should be further noted according to the following example that the same hardness of Example 6 is obtained without the use of the combination additive of the invention, i.e. where the polycarboxylic acid (oxalic acid) is eliminated.

EXAMPLE 7 Gold as potassium gold cyanide10 grams/liter Potassium citrate-70 grams/liter Potassium dihydrogen phosphate-30 grams/liter Arsenic as arsenic trioxide30 mg./liter pH adjusted to 6.5 with H PO or KOH The hardness of the deposit plated at 5 ASP was 140 Knoop.

The results of Example 6indicate that the efiect of the hardness control additive is eifective. only in the noncyanide gold electrolytes.

Various modifications of the electrolyte of the invention are contemplated within the scope of the appended claims. a a

What is claimed is:

1. An aqueous alkaline goldplating electrolyte comprising:

(a) 430 grams/liter of gold as the sulfite complex selected from the group consisting of potassium gold sulfite, sodium gold sulfite and ammonium gold sulfite,

(b) 10-200 grams/liter of a bulfering salt selected from the group consisting of alkali metal or ammonium citrates, borates', phosphates, sulfamates and pyrophosphates,

(c) 10150 grams/liter of an alkali metal sulfite selected from the group consisting of potassium sulfite and sodium sulfite,

(d) 0.0251.5 grams/liter of a non-hydroxy polycarboxylic acid selected from the group consisting of succinic, malonic and oxalic acids,

(e) 0.00250.15 gram/liter of a semi-metal brightener, as metal, selected from the group consisting'of arsenic, antimony, selenium and tellurium, the combination of the non-hydroxy polycarboxylic acid and the semi-metal brightener being in the range of a 4:1 to 20:1 ratio of non-hydroxy polycarboxylic acid to semi-metal brightener, and sufficient alkali metal hydroxide to adjust the pH of the electrolyte to 8-11.

2. A gold plating electrolyte according to claim 1, wherein the non-hydroxy polycarboxylic acid is oxalic acid and the semi-metal brightener is arsenic trioxide.

3. A gold plating electrolyte according to claim 1, wherein the combination of the non-hydroxy polycarboxylic acid and semi-metal brightener is in the ratio of 5 :1 non-hydroxy polycarboxylic acid to semi-metal brightener with the gold deposited therefrom having a hardness of to Knoop.

4. A gold plating electrolyte according to claim 1, wherein the combination of the non-hydroxy polycarboxylic acid and semi-metal brightener is in the ratio of 10:1 non-hydroxy polycarboxylic acid to semi-metal brightener with the gold deposited therefrom having a hardness below 90 Knoop.

5. A gold plating electrolyte according to claim 1, wherein the pH adjusting alkali metal hydroxide is potassium hydroxide.

References Cited UNITED STATES PATENTS 3,666,6 40 5/1972 Smith 204-44 2,967,135 1/1961 Ostrow et a1. 204-43 3,475,292 10/1969 Shoushanian 204-44 GERALD L. KAPLAN, Primary Examiner v US. Cl. X.R. 204-43 G 

